There is an increasing demand for higher image qualities in image forming apparatuses, such as copy machines, laser printers, and facsimile machines. Particularly, in full-color printing applications, there is the problem of image displacement in multi-color images.
A photosensitive drum used for forming an image by an electrophotographic technology is typically subjected to the processes of charging, latent image formation and development, image transfer, and cleaning by various units disposed around the photosensitive drum while the photosensitive drum is rotated. In order to achieve high image quality, an entire surface of the photosensitive drum needs to be uniformly charged and developed under uniform developing conditions.
Because the photosensitive drum is rotated during such processes, the photosensitive drum is required to have a high runout accuracy (which indicates the amount of variation in the distance between a rotation center and a peripheral surface of the photosensitive drum). Generally, the photosensitive drum includes a cylindrical base of a metal, such as aluminum. On an outer peripheral surface of the cylindrical base, a photosensitive layer is provided. The cylindrical base with the photosensitive layer formed thereon may be referred to as a “sleeve member”. Further, flange members are attached to end-opening portions at the ends of the sleeve member in the axial direction thereof.
The photosensitive drum is supported by, and rotated with respect to, an apparatus main body via the flange members and a shaft member engaged in axle openings provided in the flange members. Thus, the flange members need to be accurately and securely attached to the end-opening portions of the sleeve member. In order to achieve a smooth and accurate rotation of the photosensitive drum, the axle openings in the flange members need to be aligned with the central axis of the sleeve member at all times. Further, in order to allow the photosensitive drum to be rotated smoothly and without error, the flange members need to be prevented from idly rotating with respect to the sleeve member or being detached therefrom. For these purposes, the sleeve member and the flange members are typically assembled by press fitting (in combination with an adhesive, as needed).
In comparison with the base of the sleeve member, the flange members generally have lower rigidity. As a result, the flange members may be deformed at the time of press fitting, resulting in the deformation or displacement of the axle openings of the flange members. Specifically, the flange member includes a press-fitted portion that is press-fitted in the end-opening portion of the sleeve member, an axle opening portion including the axle opening, and a linking portion that extends in a direction parallel to a circular cross section of the sleeve member upon press fitting. The linking portion links the axle opening portion with the press-fitted portion. When the press-fitted portion is press-fitted in the end-opening portion of the sleeve member, an outer peripheral surface of the press-fitted portion that contacts an inner peripheral surface of the end-opening portion is subjected to stress from the inner peripheral surface of the sleeve member. When the stress is transmitted from the press-fitted portion via the linking portion to the axle opening portion, the axle opening in the axle opening portion is deformed or displaced.
When the axle opening is deformed or displaced, the position of the axle opening of the flange member with respect to the central axis of the sleeve member is displaced, thereby decreasing the runout accuracy. It has been difficult to manufacture the photosensitive drum having a high runout accuracy in a stable manner.
Japanese Laid-open Utility Model Publication 01-136959 (“Patent Document 1”) discusses a flange member having an elastic structure. However, the stress absorbing capacity of this elastic structure is not very high, so that the axle opening may be readily deformed or displaced, resulting in low runout accuracy. Further, the structure of Patent Document 1 includes areas where the sleeve member and the flange member are not in close contact with each other, resulting in the problem of idle rotation or detachment.
Japanese Laid-open Patent Publication No. 08-123251 (“Patent Document 2”) or Japanese Laid-open Patent Publication No. 10-288917 (“Patent Document 3”) discloses a flange structure in which the linking portion includes straight ribs and opening portions between the ribs. By providing the opening portions, when the outer peripheral surface of the press-fitted portion is subjected to stress from the inner peripheral surface of the sleeve member, the stress can be absorbed by deformation of the linking portion around the opening portions. In this way, the deformation or displacement of the axle opening due to the stress applied to the axle opening portion can be prevented.
However, in the disclosures of Patent Documents 2 and 3, the ribs linking the press-fitted portion and the axle-opening portion are straight-shaped. Thus, when the stress is applied to the straight ribs in a direction along the length of the ribs, the stress is not absorbed by the opening portions upon press fitting but instead directly transmitted to the axle opening portion. As a result, the axle opening is deformed or displaced, resulting in a decrease in the runout accuracy.
The above problems may arise not just in the flange members attached to the sleeve member for the photosensitive drums, but also in the case of any flange member that is press-fitted in an end-opening portion of a cylindrical member.